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7 MySQL Optimization

Optimization is a complicated task because it ultimately requires understanding of the whole system. While it may be possible to perform some local optimizations with small knowledge of your system or application, the more optimal you want your system to become the more you will have to know about it.

This chapter tries to explain and give some examples of different ways to optimize MySQL. Remember, however, that there are always some (increasingly harder) additional ways to make the system even faster.

7.1 Optimization Overview

The most important factor in making a system fast is the basic design. You also need to know what kinds of things your system will be doing, and what your bottlenecks are.

The most common bottlenecks are:

7.1.1 MySQL Design Limitations/Tradeoffs

When using the MyISAM storage engine, MySQL uses extremely fast table locking (multiple readers / single writers). The biggest problem with this table type occurs when you have a mix of a steady stream of updates and slow selects on the same table. If this is a problem with some tables, you can use another table type for these. See section 14 MySQL Table Types.

MySQL can work with both transactional and non-transactional tables. To be able to work smoothly with non-transactional tables (which can't roll back if something goes wrong), MySQL has the following rules:

For more information about this, see See section 1.8.6 How MySQL Deals with Constraints.

The above means that one should not use MySQL to check fields content, but one should do this in the application.

7.1.2 Portability

Because all SQL servers implement different parts of SQL, it takes work to write portable SQL applications. For very simple selects/inserts it is very easy, but the more you need the harder it gets. If you want an application that is fast with many databases it becomes even harder!

To make a complex application portable you need to choose a number of SQL servers that it should work with.

You can use the MySQL crash-me program/web-page http://www.mysql.com/information/crash-me.php to find functions, types, and limits you can use with a selection of database servers. Crash-me now tests far from everything possible, but it is still comprehensive with about 450 things tested.

For example, you shouldn't have column names longer than 18 characters if you want to be able to use Informix or DB2.

Both the MySQL benchmarks and crash-me programs are very database-independent. By taking a look at how we have handled this, you can get a feeling for what you have to do to write your application database-independent. The benchmarks themselves can be found in the `sql-bench' directory in the MySQL source distribution. They are written in Perl with DBI database interface (which solves the access part of the problem).

See http://www.mysql.com/information/benchmarks.html for the results from this benchmark.

As you can see in these results, all databases have some weak points. That is, they have different design compromises that lead to different behavior.

If you strive for database independence, you need to get a good feeling for each SQL server's bottlenecks. MySQL is very fast in retrieving and updating records, but will have a problem in mixing slow readers/writers on the same table. Oracle, on the other hand, has a big problem when you try to access rows that you have recently updated (until they are flushed to disk). Transaction databases in general are not very good at generating summary tables from log tables, as in this case row locking is almost useless.

To get your application really database-independent, you need to define an easy extendable interface through which you manipulate your data. As C++ is available on most systems, it makes sense to use a C++ classes interface to the databases.

If you use some specific feature for some database (like the REPLACE command in MySQL), you should code a method for the other SQL servers to implement the same feature (but slower). With MySQL you can use the /*! */ syntax to add MySQL-specific keywords to a query. The code inside /**/ will be treated as a comment (ignored) by most other SQL servers.

If high performance is more important than exactness, as in some web applications, it is possibile to create an application layer that caches all results to give you even higher performance. By letting old results 'expire' after a while, you can keep the cache reasonably fresh. This provides a method to handle high load spikes, in which case you can dynamically increase the cache and set the expire timeout higher until things get back to normal.

In this case the table creation information should contain information of the initial size of the cache and how often the table should normally be refreshed.

7.1.3 What We Have Used MySQL For

During MySQL initial development, the features of MySQL were made to fit our largest customer. They handle data warehousing for a couple of the biggest retailers in Sweden.

From all stores, we get weekly summaries of all bonus card transactions, and we are expected to provide useful information for the store owners to help them find how their advertisement campaigns are affecting their customers.

The data is quite huge (about 7 million summary transactions per month), and we have data for 4-10 years that we need to present to the users. We got weekly requests from the customers that they want to get 'instant' access to new reports from this data.

We solved this by storing all information per month in compressed 'transaction' tables. We have a set of simple macros (script) that generates summary tables grouped by different criteria (product group, customer id, store ...) from the transactional tables. The reports are web pages that are dynamically generated by a small Perl script that parses a web page, executes the SQL statements in it, and inserts the results. We would have used PHP or mod_perl instead but they were not available at that time.

For graphical data we wrote a simple tool in C that can produce GIFs based on the result of an SQL query (with some processing of the result). This is also dynamically executed from the Perl script that parses the HTML files.

In most cases a new report can simply be done by copying an existing script and modifying the SQL query in it. In some cases, we will need to add more fields to an existing summary table or generate a new one, but this is also quite simple, as we keep all transactions tables on disk. (Currently we have at least 50G of transactions tables and 200G of other customer data.)

We also let our customers access the summary tables directly with ODBC so that the advanced users can themselves experiment with the data.

We haven't had any problems handling this with quite modest Sun Ultra SPARCstation (2x200 Mhz). We recently upgraded one of our servers to a 2 CPU 400 Mhz UltraSPARC, and we are now planning to start handling transactions on the product level, which would mean a ten-fold increase of data. We think we can keep up with this by just adding more disk to our systems.

We are also experimenting with Intel-Linux to be able to get more CPU power cheaper. Now that we have the binary portable database format (new in Version 3.23), we will start to use this for some parts of the application.

Our initial feelings are that Linux will perform much better on low-to-medium load and Solaris will perform better when you start to get a high load because of extreme disk IO, but we don't yet have anything conclusive about this. After some discussion with a Linux kernel developer, this might be a side effect of Linux allocating so many resources to the batch job that the interactive performance gets very low. This makes the machine feel very slow and unresponsive while big batches are going. Hopefully this will be better handled in future Linux Kernels.

7.1.4 The MySQL Benchmark Suite

This section should contain a technical description of the MySQL benchmark suite (and crash-me), but that description is not written yet. Currently, you can get a good idea of the benchmark by looking at the code and results in the `sql-bench' directory in any MySQL source distribution.

This benchmark suite is meant to be a benchmark that will tell any user what operations a given SQL implementation performs well or poorly.

Note that this benchmark is single-threaded, so it measures the minimum time for the operations performed. We plan to add a lot of multi-threaded tests to the benchmark suite in the future.

The following tables show some comparative benchmark results for several database servers when accessed through ODBC on a Windows NT 4.0 machine.

Reading 2000000 rows by index Seconds Seconds
mysql 367 249
mysql_odbc 464
db2_odbc 1206
informix_odbc 121126
ms-sql_odbc 1634
oracle_odbc 20800
solid_odbc 877
sybase_odbc 17614
Inserting 350768 rows Seconds Seconds
mysql 381 206
mysql_odbc 619
db2_odbc 3460
informix_odbc 2692
ms-sql_odbc 4012
oracle_odbc 11291
solid_odbc 1801
sybase_odbc 4802

For the preceding tests, MySQL was run with an index cache size of 8M.

We have gathered some more benchmark results at http://www.mysql.com/information/benchmarks.html.

Note that Oracle is not included because they asked to be removed. All Oracle benchmarks have to be passed by Oracle! We believe that makes Oracle benchmarks very biased because the above benchmarks are supposed to show what a standard installation can do for a single client.

To use the benchmark suite, the following requirements must be satisified:

The benchmark suite is located in the `sql-bench' directory of MySQL source distributions. To run the benchmark tests, change location into that directory and execute the run-all-tests script: 

shell> cd sql-bench
shell> perl run-all-tests --server=server_name

server_name is one of supported servers. You can get a list of all options and supported servers by invoking run-all-tests --help.

crash-me tries to determine what features a database supports and what its capabilities and limitations are by actually running queries. For example, it determines:

We can find the result from crash-me on a lot of different databases at http://www.mysql.com/information/crash-me.php.

7.1.5 Using Your Own Benchmarks

You should definitely benchmark your application and database to find out where the bottlenecks are. By fixing it (or by replacing the bottleneck with a ``dummy module'') you can then easily identify the next bottleneck (and so on). Even if the overall performance for your application currently is acceptable, you should at least make a plan for each bottleneck, and decide how to solve it if someday you really need the extra performance.

For an example of portable benchmark programs, look at the MySQL benchmark suite. See section 7.1.4 The MySQL Benchmark Suite. You can take any program from this suite and modify it for your needs. By doing this, you can try different solutions to your problem and test which is really fastest for you.

Another free benchmark suite is the Open Source Database Benchmark, available at http://osdb.sourceforge.net/.

It is very common for a problem to occur only when the system is very heavily loaded. We have had many customers who contact us when they have a (tested) system in production and have encountered load problems. In most cases, performance problems turn out to be due to issues of basic database design (table scans are not good at high load) or problems with the operating system or libraries. Most of the time, these problems would be a lot easier to fix if the systems were not already in production.

To avoid problems like this, you should put some effort into benchmarking your whole application under the worst possible load! You can use Super Smack for this. It is available at http://www.mysql.com/Downloads/super-smack/super-smack-1.0.tar.gz. As the name suggests, it can bring your system to its knees if you ask it, so make sure to use it only on your development systems.

7.2 Optimizing SELECT Statements and Other Queries

First, one thing that affects all queries: The more complex permission system setup you have, the more overhead you get.

If you do not have any GRANT statements done, MySQL will optimize the permission checking somewhat. So if you have a very high volume it may be worth the time to avoid grants. Otherwise, more permission check results in a larger overhead.

If your problem is with some explicit MySQL function, you can always time this in the MySQL client: 

mysql> SELECT BENCHMARK(1000000,1+1);
+------------------------+
| BENCHMARK(1000000,1+1) |
+------------------------+
|                      0 |
+------------------------+
1 row in set (0.32 sec)

The above shows that MySQL can execute 1,000,000 + expressions in 0.32 seconds on a PentiumII 400MHz.

All MySQL functions should be very optimized, but there may be some exceptions, and the BENCHMARK(loop_count,expression) is a great tool to find out if this is a problem with your query.

7.2.1 EXPLAIN Syntax (Get Information About a SELECT)

    EXPLAIN tbl_name
or  EXPLAIN SELECT select_options

EXPLAIN tbl_name is a synonym for DESCRIBE tbl_name or SHOW COLUMNS FROM tbl_name.

When you precede a SELECT statement with the keyword EXPLAIN, MySQL explains how it would process the SELECT, providing information about how tables are joined and in which order.

With the help of EXPLAIN, you can see when you must add indexes to tables to get a faster SELECT that uses indexes to find the records.

You should frequently run ANALYZE TABLE to update table statistics such as cardinality of keys which can affect the choices the optimizer makes. See section 13.5.2.1 ANALYZE TABLE Syntax.

You can also see if the optimizer joins the tables in an optimal order. To force the optimizer to use a specific join order for a SELECT statement, add a STRAIGHT_JOIN clause.

For non-simple joins, EXPLAIN returns a row of information for each table used in the SELECT statement. The tables are listed in the order they would be read. MySQL resolves all joins using a single-sweep multi-join method. This means that MySQL reads a row from the first table, then finds a matching row in the second table, then in the third table and so on. When all tables are processed, it outputs the selected columns and backtracks through the table list until a table is found for which there are more matching rows. The next row is read from this table and the process continues with the next table.

In MySQL version 4.1 the EXPLAIN output was changed to work better with constructs like UNION statements, subqueries and derived tables. Most notable is the addition of two new columns: id and select_type.

Output from EXPLAIN consists of the following columns:

id
SELECT identifier, the sequential number of this SELECT within the query.
select_type
Type of SELECT clause, which can be any of the following:
SIMPLE
Simple SELECT (not using UNION or subqueries).
PRIMARY
Outermost SELECT.
UNION
Second and further SELECT statements in a UNION.
DEPENDENT UNION
Second and further SELECT statements in a UNION, dependent on outer subquery.
SUBQUERY
First SELECT in subquery.
DEPENDENT SUBQUERY
First SELECT, dependent on outer subquery.
DERIVED
Derived table SELECT (subquery in FROM clause).
table
The table to which the row of output refers.
type
The join type. The different join types are listed here, ordered from best to worst type:
system
The table has only one row (= system table). This is a special case of the const join type.
const
The table has at most one matching row, which will be read at the start of the query. Because there is only one row, values from the column in this row can be regarded as constants by the rest of the optimizer. const tables are very fast as they are read only once! const is used when you compare all parts of a PRIMARY/UNIQUE key with constants: 
SELECT * FROM const_table WHERE primary_key=1;

SELECT * FROM const_table
WHERE primary_key_part1=1 AND primary_key_part2=2;
eq_ref
One row will be read from this table for each combination of rows from the previous tables. This is the best possible join type, other than the const types. It is used when all parts of an index are used by the join and the index is UNIQUE or a PRIMARY KEY. eq_ref can be used for indexed columns that is compared with the = operator. The compared item may be a constant or an expression that uses columns from tables that are read before this table. In the following examples, ref_table will be able to use eq_ref 
SELECT * FROM ref_table,other_table
WHERE ref_table.key_column=other_table.column;

SELECT * FROM ref_table,other_table
WHERE ref_table.key_column_part1=other_table.column
AND ref_table.key_column_part2=1;
ref
All rows with matching index values will be read from this table for each combination of rows from the previous tables. ref is used if the join uses only a leftmost prefix of the key, or if the key is not UNIQUE or a PRIMARY KEY (in other words, if the join cannot select a single row based on the key value). If the key that is used matches only a few rows, this join type is good. ref can be used for indexed columns that is compared with the = operator. In the following examples, ref_table will be able to use ref. 
SELECT * FROM ref_table WHERE key_column=expr;

SELECT * FROM ref_table,other_table
WHERE ref_table.key_column=other_table.column;

SELECT * FROM ref_table,other_table
WHERE ref_table.key_column_part1=other_table.column
AND ref_table.key_column_part2=1;
ref_or_null
Like ref, but with the addition that we will do an extra search for rows with NULL. See section 7.2.6 How MySQL Optimizes IS NULL. 
SELECT * FROM ref_table WHERE key_column=expr OR key_column IS NULL;
This join type optimization is new for MySQL 4.1.1 and is mostly used when resolving subqueries.
unique_subquery
This type replaces ref for some IN subqueries of the following form: 
value IN (SELECT primary_key FROM single_table WHERE some_exp)
unique_subquery is just an index lookup function that replaces the subquery completely for better efficiency.
index_subquery
Like unique_subquery, this type replaces IN subqueries, but it works for non-unique indexes: 
value IN (SELECT key_field FROM single_table WHERE some_exp)
range
Only rows that are in a given range will be retrieved, using an index to select the rows. The key column indicates which index is used. The key_len contains the longest key part that was used. The ref column will be NULL for this type. range can be used for when an key column is compared to a constant with =, <>, >, >=, <, <=, IS NULL, <=>, BETWEEN and IN. 
SELECT * FROM range_table WHERE key_column = 10;

SELECT * FROM range_table WHERE key_column BETWEEN 10 and 20;

SELECT * FROM range_table WHERE key_column IN (10,20,30);

SELECT * FROM range_table WHERE key_part1= 10 and key_part2 IN (10,20,30);
index
This is the same as ALL, except that only the index tree is scanned. This is usually faster than ALL, as the index file is usually smaller than the datafile. This can be used when the query only uses columns that are part of one index.
ALL
A full table scan will be done for each combination of rows from the previous tables. This is normally not good if the table is the first table not marked const, and usually very bad in all other cases. You normally can avoid ALL by adding more indexes, so that the row can be retrieved based on constant values or column values from earlier tables.
possible_keys
The possible_keys column indicates which indexes MySQL could use to find the rows in this table. Note that this column is totally independent of the order of the tables. That means that some of the keys in possible_keys may not be usable in practice with the generated table order. If this column is NULL, there are no relevant indexes. In this case, you may be able to improve the performance of your query by examining the WHERE clause to see if it refers to some column or columns that would be suitable for indexing. If so, create an appropriate index and check the query with EXPLAIN again. See section 13.2.2 ALTER TABLE Syntax. To see what indexes a table has, use SHOW INDEX FROM tbl_name.
key
The key column indicates the key (index) that MySQL actually decided to use. The key is NULL if no index was chosen. To force MySQL to use an key listed in the possible_keys column, use USE KEY/IGNORE KEY in your query. See section 13.1.7 SELECT Syntax. Also, running myisamchk --analyze (see section 5.5.2.1 myisamchk Invocation Syntax) or ANALYZE TABLE (see section 13.5.2.1 ANALYZE TABLE Syntax) on the table will help the optimizer choose better indexes.
key_len
The key_len column indicates the length of the key that MySQL decided to use. The length is NULL if the key is NULL. Note that this tells us how many parts of a multi-part key MySQL will actually use.
ref
The ref column shows which columns or constants are used with the key to select rows from the table.
rows
The rows column indicates the number of rows MySQL believes it must examine to execute the query.
Extra
This column contains additional information of how MySQL will resolve the query. Here is an explanation of the different text strings that can be found in this column:
Distinct
MySQL will not continue searching for more rows for the current row combination after it has found the first matching row.
Not exists
MySQL was able to do a LEFT JOIN optimization on the query and will not examine more rows in this table for the previous row combination after it finds one row that matches the LEFT JOIN criteria. Here is an example for this: 
SELECT * FROM t1 LEFT JOIN t2 ON t1.id=t2.id
WHERE t2.id IS NULL;
Assume that t2.id is defined with NOT NULL. In this case MySQL will scan t1 and look up the rows in t2 through t1.id. If MySQL finds a matching row in t2, it knows that t2.id can never be NULL, and will not scan through the rest of the rows in t2 that has the same id. In other words, for each row in t1, MySQL only needs to do a single lookup in t2, independent of how many matching rows there are in t2.
range checked for each record (index map: #)
MySQL didn't find a real good index to use. It will, instead, for each row combination in the preceding tables, do a check on which index to use (if any), and use this index to retrieve the rows from the table. This isn't very fast but is faster than having to do a join without an index.
Using filesort
MySQL will need to do an extra pass to find out how to retrieve the rows in sorted order. The sort is done by going through all rows according to the join type and storing the sort key + pointer to the row for all rows that match the WHERE. Then the keys are sorted. Finally the rows are retrieved in sorted order.
Using index
The column information is retrieved from the table using only information in the index tree without having to do an additional seek to read the actual row. This can be done when all the used columns for the table are part of the same index.
Using temporary
To resolve the query MySQL will need to create a temporary table to hold the result. This typically happens if you do an ORDER BY on a different column set than you did a GROUP BY on.
Using where
A WHERE clause will be used to restrict which rows will be matched against the next table or sent to the client. If you don't have this information and the table is of type ALL or index, you may have something wrong in your query (if you don't intend to fetch/examine all rows from the table).
If you want to get your queries as fast as possible, you should look out for Using filesort and Using temporary.

You can get a good indication of how good a join is by multiplying all values in the rows column of the EXPLAIN output. This should tell you roughly how many rows MySQL must examine to execute the query. This number is also used when you restrict queries with the max_join_size variable. See section 7.5.2 Tuning Server Parameters.

The following example shows how a JOIN can be optimized progressively using the information provided by EXPLAIN.

Suppose you have the SELECT statement shown here, that you examine using EXPLAIN: 

EXPLAIN SELECT tt.TicketNumber, tt.TimeIn,
            tt.ProjectReference, tt.EstimatedShipDate,
            tt.ActualShipDate, tt.ClientID,
            tt.ServiceCodes, tt.RepetitiveID,
            tt.CurrentProcess, tt.CurrentDPPerson,
            tt.RecordVolume, tt.DPPrinted, et.COUNTRY,
            et_1.COUNTRY, do.CUSTNAME
        FROM tt, et, et AS et_1, do
        WHERE tt.SubmitTime IS NULL
            AND tt.ActualPC = et.EMPLOYID
            AND tt.AssignedPC = et_1.EMPLOYID
            AND tt.ClientID = do.CUSTNMBR;

For this example, assume that:

Initially, before any optimizations have been performed, the EXPLAIN statement produces the following information: 

table type possible_keys                key  key_len ref  rows  Extra
et    ALL  PRIMARY                      NULL NULL    NULL 74
do    ALL  PRIMARY                      NULL NULL    NULL 2135
et_1  ALL  PRIMARY                      NULL NULL    NULL 74
tt    ALL  AssignedPC,ClientID,ActualPC NULL NULL    NULL 3872
      range checked for each record (key map: 35)

Because type is ALL for each table, this output indicates that MySQL is generating a Cartesian product of all the tables! This will take quite a long time, as the product of the number of rows in each table must be examined! For the case at hand, this is 74 * 2135 * 74 * 3872 = 45,268,558,720 rows. If the tables were bigger, you can only imagine how long it would take.

One problem here is that MySQL can't (yet) use indexes on columns efficiently if they are declared differently. In this context, VARCHAR and CHAR are the same unless they are declared as different lengths. Because tt.ActualPC is declared as CHAR(10) and et.EMPLOYID is declared as CHAR(15), there is a length mismatch.

To fix this disparity between column lengths, use ALTER TABLE to lengthen ActualPC from 10 characters to 15 characters: 

mysql> ALTER TABLE tt MODIFY ActualPC VARCHAR(15);

Now tt.ActualPC and et.EMPLOYID are both VARCHAR(15). Executing the EXPLAIN statement again produces this result: 

table type   possible_keys   key     key_len ref         rows    Extra
tt    ALL    AssignedPC,ClientID,ActualPC NULL NULL NULL 3872    Using where
do    ALL    PRIMARY         NULL    NULL    NULL        2135
      range checked for each record (key map: 1)
et_1  ALL    PRIMARY         NULL    NULL    NULL        74
      range checked for each record (key map: 1)
et    eq_ref PRIMARY         PRIMARY 15      tt.ActualPC 1

This is not perfect, but is much better (the product of the rows values is now less by a factor of 74). This version is executed in a couple of seconds.

A second alteration can be made to eliminate the column length mismatches for the tt.AssignedPC = et_1.EMPLOYID and tt.ClientID = do.CUSTNMBR comparisons: 

mysql> ALTER TABLE tt MODIFY AssignedPC VARCHAR(15),
    ->                MODIFY ClientID   VARCHAR(15);

Now EXPLAIN produces the output shown here: 

table type   possible_keys   key      key_len ref           rows Extra
et    ALL    PRIMARY         NULL     NULL    NULL          74
tt    ref    AssignedPC,     ActualPC 15      et.EMPLOYID   52   Using where
             ClientID,
             ActualPC
et_1  eq_ref PRIMARY         PRIMARY  15      tt.AssignedPC 1
do    eq_ref PRIMARY         PRIMARY  15      tt.ClientID   1

This is almost as good as it can get.

The remaining problem is that, by default, MySQL assumes that values in the tt.ActualPC column are evenly distributed, and that isn't the case for the tt table. Fortunately, it is easy to tell MySQL about this: 

shell> myisamchk --analyze PATH_TO_MYSQL_DATABASE/tt
shell> mysqladmin refresh

Now the join is perfect, and EXPLAIN produces this result: 

table type   possible_keys key     key_len ref           rows Extra
tt    ALL    AssignedPC    NULL    NULL    NULL          3872 Using where
             ClientID,
             ActualPC
et    eq_ref PRIMARY       PRIMARY 15      tt.ActualPC   1
et_1  eq_ref PRIMARY       PRIMARY 15      tt.AssignedPC 1
do    eq_ref PRIMARY       PRIMARY 15      tt.ClientID   1

Note that the rows column in the output from EXPLAIN is an educated guess from the MySQL join optimizer. To optimize a query, you should check if the numbers are even close to the truth. If not, you may get better performance by using STRAIGHT_JOIN in your SELECT statement and trying to list the tables in a different order in the FROM clause.

7.2.2 Estimating Query Performance

In most cases you can estimate the performance by counting disk seeks. For small tables, you can usually find the row in 1 disk seek (as the index is probably cached). For bigger tables, you can estimate that (using B++ tree indexes) you will need: log(row_count) / log(index_block_length / 3 * 2 / (index_length + data_pointer_length)) + 1 seeks to find a row.

In MySQL an index block is usually 1024 bytes and the data pointer is usually 4 bytes. A 500,000 row table with an index length of 3 (medium integer) gives you: log(500,000)/log(1024/3*2/(3+4)) + 1 = 4 seeks.

As the above index would require about 500,000 * 7 * 3/2 = 5.2M, (assuming that the index buffers are filled to 2/3, which is typical) you will probably have much of the index in memory and you will probably only need 1-2 calls to read data from the OS to find the row.

For writes, however, you will need 4 seek requests (as above) to find where to place the new index and normally 2 seeks to update the index and write the row.

Note that the above doesn't mean that your application will slowly degenerate by log N! As long as everything is cached by the OS or SQL server things will only go marginally slower while the table gets bigger. After the data gets too big to be cached, things will start to go much slower until your applications is only bound by disk-seeks (which increase by log N). To avoid this, increase the index cache as the data grows. See section 7.5.2 Tuning Server Parameters.

7.2.3 Speed of SELECT Queries

In general, when you want to make a slow SELECT ... WHERE faster, the first thing to check is whether you can add an index. See section 7.4.3 How MySQL Uses Indexes. All references between different tables should usually be done with indexes. You can use the EXPLAIN command to determine which indexes are used for a SELECT. See section 7.2.1 EXPLAIN Syntax (Get Information About a SELECT).

Some general tips:

7.2.4 How MySQL Optimizes WHERE Clauses

The WHERE optimizations are put in the SELECT part here because they are mostly used with SELECT, but the same optimizations apply for WHERE in DELETE and UPDATE statements.

Also note that this section is incomplete. MySQL does many optimizations, and we have not had time to document them all.

Some of the optimizations performed by MySQL are listed here:

Some examples of queries that are very fast: 

mysql> SELECT COUNT(*) FROM tbl_name;
mysql> SELECT MIN(key_part1),MAX(key_part1) FROM tbl_name;
mysql> SELECT MAX(key_part2) FROM tbl_name
    ->        WHERE key_part_1=constant;
mysql> SELECT ... FROM tbl_name
    ->        ORDER BY key_part1,key_part2,... LIMIT 10;
mysql> SELECT ... FROM tbl_name
    ->        ORDER BY key_part1 DESC,key_part2 DESC,... LIMIT 10;

The following queries are resolved using only the index tree (assuming the indexed columns are numeric): 

mysql> SELECT key_part1,key_part2 FROM tbl_name WHERE key_part1=val;
mysql> SELECT COUNT(*) FROM tbl_name
    ->        WHERE key_part1=val1 AND key_part2=val2;
mysql> SELECT key_part2 FROM tbl_name GROUP BY key_part1;

The following queries use indexing to retrieve the rows in sorted order without a separate sorting pass: 

mysql> SELECT ... FROM tbl_name
    ->            ORDER BY key_part1,key_part2,... ;
mysql> SELECT ... FROM tbl_name
    ->            ORDER BY key_part1 DESC,key_part2 DESC,... ;

7.2.5 How MySQL Optimizes OR Clauses

The Merge Index method is used to retrieve rows with several ref, ref_or_null or range scans and merge the results into one. This method is employed when the table condition is a disjunction of conditions for which ref, ref_or_null, or range could be used with different keys. The key column contains a list of used indexes. key_len contains a list of the longest key parts of the used indexes.

Example: 

SELECT * FROM table WHERE key1column = 10 OR key2column = 20;

SELECT * FROM table WHERE 
  (key1column = 10 OR key2column = 20) AND nonkeycolumn=30;

SELECT * FROM t1,t2 WHERE 
  (t1.key1 IN (1,2) OR t1.key2 LIKE 'value%') AND t2.key1=t1.somefield

SELECT * FROM t1,t2 WHERE 
  t1.key1=1 AND (t2.key1=t1.somefield OR t2.key2=t1.somefield2)

This ``join'' type optimization is new in MySQL 5.0.0, and represents a significant change in behaviour with regard to indexes since the old rule was that the server is only ever able to use at most one index for each referenced table.

7.2.6 How MySQL Optimizes IS NULL

MySQL can do the same optimization on column IS NULL as it can do with column = constant_value. For example, MySQL can use indexes and ranges to search for NULL with IS NULL. 

SELECT * FROM table_name WHERE key_col IS NULL;

SELECT * FROM table_name WHERE key_col <=> NULL;

SELECT * FROM table_name WHERE key_col=# OR key_col=# OR key_col IS NULL

If you use column_name IS NULL on a NOT NULL in a WHERE clause on table that is not used OUTER JOIN that expression will be optimized away.

MySQL 4.1.1 can additionally optimize the combination column = expr AND column IS NULL, an form that is common in resolved sub queries. EXPLAIN will show ref_or_null when this optimization is used.

This optimization can handle one IS NULL for any key part.

Some examples of queries that are optimized (assuming key on t2 (a,b)): 

SELECT * FROM t1 WHERE t1.a=expr OR t1.a IS NULL;

SELECT * FROM t1,t2 WHERE t1.a=t2.a OR t2.a IS NULL;

SELECT * FROM t1,t2 WHERE (t1.a=t2.a OR t2.a IS NULL) AND t2.b=t1.b;

SELECT * FROM t1,t2 WHERE t1.a=t2.a AND (t2.b=t1.b OR t2.b IS NULL);

SELECT * FROM t1,t2 WHERE (t1.a=t2.a AND t2.a IS NULL AND ...) OR (t1.a=t2.a AND t2.a IS NULL AND ...);

ref_or_null works by first doing a read on the reference key and after that a separate search after rows with NULL key.

Note that the optimization can only handle one IS NULL level. 

SELECT * FROM t1,t2 where (t1.a=t2.a AND t2.a IS NULL) OR (t1.b=t2.b AND t2.b IS NULL);

Int the above case MySQL will only use key lookups on the part (t1.a=t2.a AND t2.a IS NULL) and not be able to use the key part on b.

7.2.7 How MySQL Optimizes DISTINCT

DISTINCT combined with ORDER BY will in many cases need a temporary table.

Note that as DISTINCT may use GROUP BY, you should be aware of how MySQL works with in fields in ORDER BY or HAVING that are not part of the selected fields. See section 12.7.3 GROUP BY with Hidden Fields.

When combining LIMIT row_count with DISTINCT, MySQL will stop as soon as it finds row_count unique rows.

If you don't use columns from all used tables, MySQL will stop the scanning of the not used tables as soon as it has found the first match. 

SELECT DISTINCT t1.a FROM t1,t2 where t1.a=t2.a;

In this case, assuming t1 is used before t2 (check with EXPLAIN), then MySQL will stop reading from t2 (for that particular row in t1) when the first row in t2 is found.

7.2.8 How MySQL Optimizes LEFT JOIN and RIGHT JOIN

A LEFT JOIN B join_condition in MySQL is implemented as follows:

RIGHT JOIN is implemented analogously to LEFT JOIN.

The table read order forced by LEFT JOIN and STRAIGHT JOIN will help the join optimizer (which calculates in which order tables should be joined) to do its work much more quickly, as there are fewer table permutations to check.

Note that the above means that if you do a query of type: 

SELECT * FROM a,b LEFT JOIN c ON (c.key=a.key) LEFT JOIN d (d.key=a.key)
         WHERE b.key=d.key

MySQL will do a full scan on b as the LEFT JOIN will force it to be read before d.

The fix in this case is to change the query to: 

SELECT * FROM b,a LEFT JOIN c ON (c.key=a.key) LEFT JOIN d (d.key=a.key)
         WHERE b.key=d.key

Starting from 4.0.14, MySQL does the following LEFT JOIN optimization:

If the WHERE condition is always be false for the generated NULL row, the LEFT JOIN is changed to a normal join.

For example, in the following query the WHERE clause would be false if t2.column would be NULL so it's safe to convert to a normal join. 

SELECT * FROM t1 LEFT t2 ON (column) WHERE t2.column2 =5;
->
SELECT * FROM t1,t2 WHERE t2.column2=5 AND t1.column=t2.column;

This can be made faster as MySQL can now use table t2 before table t1 if this would result in a better query plan. To force a specific table order, use STRAIGHT JOIN.

7.2.9 How MySQL Optimizes ORDER BY

In some cases MySQL can uses index to satisfy an ORDER BY or GROUP BY request without doing any extra sorting.

The index can also be used even if the ORDER BY doesn't match the index exactly, as long as all the unused index parts and all the extra are ORDER BY columns are constants in the WHERE clause. The following queries will use the index to resolve the ORDER BY / GROUP BY part: 

SELECT * FROM t1 ORDER BY key_part1,key_part2,...
SELECT * FROM t1 WHERE key_part1=constant ORDER BY key_part2
SELECT * FROM t1 WHERE key_part1=constant GROUP BY key_part2
SELECT * FROM t1 ORDER BY key_part1 DESC,key_part2 DESC
SELECT * FROM t1 WHERE key_part1=1 ORDER BY key_part1 DESC,key_part2 DESC

Some cases where MySQL can not use indexes to resolve the ORDER BY: (Note that MySQL will still use indexes to find the rows that matches the WHERE clause):

In the cases where MySQL have to sort the result, it uses the following algorithm:

You can with EXPLAIN SELECT ... ORDER BY check if MySQL can use indexes to resolve the query. If you get Using filesort in the extra column, then MySQL can't use indexes to resolve the ORDER BY. See section 7.2.1 EXPLAIN Syntax (Get Information About a SELECT).

If you want to have a higher ORDER BY speed, you should first see if you can get MySQL to use indexes instead of having to do an extra sorting phase. If this is not possible, then you can do:

By default, MySQL sorts all GROUP BY x,y[,...] queries as if you specified ORDER BY x,y[,...] in the query as well. If you include the ORDER BY clause explicitly, MySQL optimizes it away without any speed penalty, though the sorting still occurs. If a query includes GROUP BY but you want to avoid the overhead of sorting the result, you can supress sorting by specifying ORDER BY NULL: 

INSERT INTO foo SELECT a,COUNT(*) FROM bar GROUP BY a ORDER BY NULL;

7.2.10 How MySQL Optimizes LIMIT

In some cases MySQL will handle the query differently when you are using LIMIT row_count and not using HAVING:

7.2.11 Speed of INSERT Queries

The time to insert a record consists approximately of:

where the numbers are somewhat proportional to the overall time. This does not take into consideration the initial overhead to open tables (which is done once for each concurrently running query).

The size of the table slows down the insertion of indexes by log N (B-trees).

Some ways to speed up inserts:

To get some more speed for both LOAD DATA INFILE and INSERT, enlarge the key buffer. See section 7.5.2 Tuning Server Parameters.

7.2.12 Speed of UPDATE Queries

Update queries are optimized as a SELECT query with the additional overhead of a write. The speed of the write is dependent on the size of the data that is being updated and the number of indexes that are updated. Indexes that are not changed will not be updated.

Also, another way to get fast updates is to delay updates and then do many updates in a row later. Doing many updates in a row is much quicker than doing one at a time if you lock the table.

Note that, with dynamic record format, updating a record to a longer total length may split the record. So if you do this often, it is very important to OPTIMIZE TABLE sometimes. See section 13.5.2.5 OPTIMIZE TABLE Syntax.

7.2.13 Speed of DELETE Queries

If you want to delete all rows in the table, you should use TRUNCATE TABLE table_name. See section 13.1.9 TRUNCATE Syntax.

The time to delete a record is exactly proportional to the number of indexes. To delete records more quickly, you can increase the size of the index cache. See section 7.5.2 Tuning Server Parameters.

7.2.14 Other Optimization Tips

Unsorted tips for faster systems:

7.3 Locking Issues

7.3.1 How MySQL Locks Tables

You can find a discussion about different locking methods in the appendix. See section D.4 Locking methods.

All locking in MySQL is deadlock-free, except for InnoDB and BDB type tables. This is managed by always requesting all needed locks at once at the beginning of a query and always locking the tables in the same order.

InnoDB type tables automatically acquire their row locks and BDB type tables their page locks during the processing of SQL statements, not at the start of the transaction.

The locking method MySQL uses for WRITE locks works as follows:

The locking method MySQL uses for READ locks works as follows:

When a lock is released, the lock is made available to the threads in the write lock queue, then to the threads in the read lock queue.

This means that if you have many updates on a table, SELECT statements will wait until there are no more updates.

To work around this for the case where you want to do many INSERT and SELECT operations on a table, you can insert rows in a temporary table and update the real table with the records from the temporary table once in a while.

This can be done with the following code: 

mysql> LOCK TABLES real_table WRITE, insert_table WRITE;
mysql> INSERT INTO real_table SELECT * FROM insert_table;
mysql> TRUNCATE TABLE insert_table;
mysql> UNLOCK TABLES;

You can use the LOW_PRIORITY options with INSERT, UPDATE or DELETE or HIGH_PRIORITY with SELECT if you want to prioritize retrieval in some specific cases. You can also start mysqld with --low-priority-updates to get the same behavior.

Using SQL_BUFFER_RESULT can also help making table locks shorter. See section 13.1.7 SELECT Syntax.

You could also change the locking code in `mysys/thr_lock.c' to use a single queue. In this case, write locks and read locks would have the same priority, which might help some applications.

7.3.2 Table Locking Issues

The table locking code in MySQL is deadlock free.

MySQL uses table locking (instead of row locking or column locking) on all table types, except InnoDB and BDB tables, to achieve a very high lock speed. For large tables, table locking is much better than row locking for most applications, but there are some pitfalls.

For InnoDB and BDB tables, MySQL only uses table locking if you explicitly lock the table with LOCK TABLES. For these table types we recommend you to not use LOCK TABLES at all, because InnoDB uses automatic row level locking and BDB uses page level locking to ensure transaction isolation.

As of MySQL Version 3.23.7 (3.23.25 for Windows), you can insert rows into MyISAM tables at the same time other threads are reading from the table. Note that currently this only works if there are no holes resulting from deleted rows in the table at the time the insert is made. When all holes has been filled with new data, concurrent inserts will automatically be enabled again.

Table locking enables many threads to read from a table at the same time, but if a thread wants to write to a table, it must first get exclusive access. During the update, all other threads that want to access this particular table will wait until the update is ready.

As updates on tables normally are considered to be more important than SELECT, all statements that update a table have higher priority than statements that retrieve information from a table. This should ensure that updates are not 'starved' because one issues a lot of heavy queries against a specific table. (You can change this by using LOW_PRIORITY with the statement that does the update or HIGH_PRIORITY with the SELECT statement.)

Starting from MySQL Version 3.23.7 one can use the max_write_lock_count variable to force MySQL to temporary give all SELECT statements, that wait for a table, a higher priority after a specific number of inserts on a table.

Table locking is, however, not very good under the following scenario:

Some possible solutions to this problem are:

7.4 Optimizing Database Structure

7.4.1 Design Choices

MySQL keeps row data and index data in separate files. Many (almost all) other databases mix row and index data in the same file. We believe that the MySQL choice is better for a very wide range of modern systems.

Another way to store the row data is to keep the information for each column in a separate area (examples are SDBM and Focus). This will cause a performance hit for every query that accesses more than one column. Because this degenerates so quickly when more than one column is accessed, we believe that this model is not good for general purpose databases.

The more common case is that the index and data are stored together (as in Oracle/Sybase et al). In this case you will find the row information at the leaf page of the index. The good thing with this layout is that it, in many cases, depending on how well the index is cached, saves a disk read. The bad things with this layout are:

7.4.2 Get Your Data as Small as Possible

One of the most basic optimization is to get your data (and indexes) to take as little space on the disk (and in memory) as possible. This can give huge improvements because disk reads are faster and normally less main memory will be used. Indexing also takes less resources if done on smaller columns.

MySQL supports a lot of different table types and row formats. Choosing the right table format may give you a big performance gain. See section 14 MySQL Table Types.

You can get better performance on a table and minimise storage space using the techniques listed here:

7.4.3 How MySQL Uses Indexes

Indexes are used to find rows with specific column values fast. Without an index MySQL has to start with the first record and then read through the whole table to find the relevant rows. The bigger the table, the more this costs. If the table has an index for the columns in question, MySQL can quickly get a position to seek to in the middle of the datafile without having to look at all the data. If a table has 1000 rows, this is at least 100 times faster than reading sequentially. Note that if you need to access almost all 1000 rows, it is faster to read sequentially, because that minimises disk seeks.

All MySQL indexes (PRIMARY KEY, UNIQUE, and INDEX) are stored in B-trees. Strings are automatically prefix- and end-space compressed. See section 13.2.4 CREATE INDEX Syntax.

Indexes are used in the following ways:

Suppose you issue the following SELECT statement: 

mysql> SELECT * FROM tbl_name WHERE col1=val1 AND col2=val2;

If a multiple-column index exists on col1 and col2, the appropriate rows can be fetched directly. If separate single-column indexes exist on col1 and col2, the optimizer tries to find the most restrictive index by deciding which index will find fewer rows and using that index to fetch the rows.

If the table has a multiple-column index, any leftmost prefix of the index can be used by the optimizer to find rows. For example, if you have a three-column index on (col1, col2, col3), you have indexed search capabilities on (col1), (col1, col2), and (col1, col2, col3).

MySQL can't use a partial index if the columns don't form a leftmost prefix of the index. Suppose you have the SELECT statements shown here: 

mysql> SELECT * FROM tbl_name WHERE col1=val1;
mysql> SELECT * FROM tbl_name WHERE col2=val2;
mysql> SELECT * FROM tbl_name WHERE col2=val2 AND col3=val3;

If an index exists on (col1, col2, col3), only the first of the preceding queries uses the index. The second and third queries do involve indexed columns, but (col2) and (col2, col3) are not leftmost prefixes of (col1, col2, col3).

MySQL also uses indexes for LIKE comparisons if the argument to LIKE is a constant string that doesn't start with a wildcard character. For example, the following SELECT statements use indexes: 

mysql> SELECT * FROM tbl_name WHERE key_col LIKE "Patrick%";
mysql> SELECT * FROM tbl_name WHERE key_col LIKE "Pat%_ck%";

In the first statement, only rows with "Patrick" <= key_col < "Patricl" are considered. In the second statement, only rows with "Pat" <= key_col < "Pau" are considered.

The following SELECT statements will not use indexes: 

mysql> SELECT * FROM tbl_name WHERE key_col LIKE "%Patrick%";
mysql> SELECT * FROM tbl_name WHERE key_col LIKE other_col;

In the first statement, the LIKE value begins with a wildcard character. In the second statement, the LIKE value is not a constant.

MySQL 4.0 does another optimization on LIKE. If you use ... LIKE "%string%" and string is longer than 3 characters, MySQL will use the Turbo Boyer-Moore algorithm to initialize the pattern for the string and then use this pattern to perform the search quicker.

Searching using column_name IS NULL will use indexes if column_name is an index.

MySQL normally uses the index that finds the smallest number of rows. An index is used for columns that you compare with the following operators: =, >, >=, <, <=, BETWEEN, or a LIKE with a pattern that begins with a non-wildcard prefix like 'something%'.

Any index that doesn't span all AND levels in the WHERE clause is not used to optimize the query. In other words: To be able to use an index, a prefix of the index must be used in every AND group.

The following WHERE clauses use indexes: 

... WHERE index_part1=1 AND index_part2=2 AND other_column=3
... WHERE index=1 OR A=10 AND index=2      /* index = 1 OR index = 2 */
... WHERE index_part1='hello' AND index_part_3=5
          /* optimized like "index_part1='hello'" */
... WHERE index1=1 AND index2=2 OR index1=3 AND index3=3;
          /* Can use index on index1 but not on index2 or index 3 */

These WHERE clauses do not use indexes: 

... WHERE index_part2=1 AND index_part3=2  /* index_part_1 is not used */
... WHERE index=1 OR A=10                  /* Index is not used in
                                                        both AND parts */
... WHERE index_part1=1 OR index_part2=10  /* No index spans all rows  */

Note that sometime MySQL will not use an index, even if one is available. One instance of this is when use of the index would require MySQL to access more than 30% of the rows in the table. (In this case a table scan is probably much faster, as it will require many fewer seeks.) However, if such a query uses LIMIT to only retrieve part of the rows, MySQL will use an index anyway, as it can much more quickly find the few rows to return in the result.

7.4.4 Column Indexes

All MySQL column types can be indexed. Use of indexes on the relevant columns is the best way to improve the performance of SELECT operations.

The maximum number of indexes per table and the maximum index length is defined per storage engine. See section 14 MySQL Table Types. All storage engines support a minimum of 16 indexes per table and a minimum total index length of 256 bytes.

For CHAR and VARCHAR columns, you can index a prefix of a column. This is much faster and requires less disk space than indexing the whole column. The syntax to use in the CREATE TABLE statement to index a column prefix looks like this: 

INDEX index_name (col_name(length))

The example here creates an index for the first 10 characters of the name column: 

mysql> CREATE TABLE test (
    ->        name CHAR(200) NOT NULL,
    ->        INDEX index_name (name(10)));

For BLOB and TEXT columns, you must index a prefix of the column. The prefix may be up to 255 bytes long.

In MySQL Version 3.23.23 or later, you can also create special FULLTEXT indexes. They are used for full-text search. Only the MyISAM table type supports FULLTEXT indexes and only for CHAR, VARCHAR, and TEXT columns. Indexing always happens over the entire column and partial (prefix) indexing is not supported. See section 13.7 MySQL Full-text Search for details.

7.4.5 Multiple-Column Indexes

MySQL can create indexes on multiple columns. An index may consist of up to 15 columns. (On CHAR and VARCHAR columns you can also use a prefix of the column as a part of an index.)

A multiple-column index can be considered a sorted array containing values that are created by concatenating the values of the indexed columns.

MySQL uses multiple-column indexes in such a way that queries are fast when you specify a known quantity for the first column of the index in a WHERE clause, even if you don't specify values for the other columns.

Suppose a table has the following specification: 

mysql> CREATE TABLE test (
    ->       id INT NOT NULL,
    ->       last_name CHAR(30) NOT NULL,
    ->       first_name CHAR(30) NOT NULL,
    ->       PRIMARY KEY (id),
    ->       INDEX name (last_name,first_name));

Then the index name is an index over last_name and first_name. The index can used for queries that specify values in a known range for last_name, or for both last_name and first_name. Therefore, the name index will be used in the following queries: 

mysql> SELECT * FROM test WHERE last_name="Widenius";

mysql> SELECT * FROM test WHERE last_name="Widenius"
    ->                    AND first_name="Michael";

mysql> SELECT * FROM test WHERE last_name="Widenius"
    ->                    AND (first_name="Michael" OR first_name="Monty");

mysql> SELECT * FROM test WHERE last_name="Widenius"
    ->                    AND first_name >="M" AND first_name < "N";

However, the name index will not be used in the following queries: 

mysql> SELECT * FROM test WHERE first_name="Michael";

mysql> SELECT * FROM test WHERE last_name="Widenius"
    ->                    OR first_name="Michael";

For more information on the manner in which MySQL uses indexes to improve query performance, see section 7.4.3 How MySQL Uses Indexes.

7.4.6 The MyISAM Key Cache

To minimize disk I/O, the MyISAM storage engine employs a strategy that is used by many database management systems. It exploits a cache mechanism to keep the most frequently accessed table blocks in memory:

This section first describes the basic operation of the MyISAM key cache. Then it discusses changes made in MySQL 4.1 that improve key cache performance and that allow you control over cache operation:

The key cache mechanism also is used for ISAM tables, which use B-tree indexes. However, the significance of this fact is on the wane. ISAM table use has been decreasing since MySQL 3.23 when MyISAM was introduced. MySQL 4.1 carries this trend further; the ISAM storage engine is disabled by default.

You can control the size of the key cache by means of the key_buffer_size system variable. If this variable is set equal to zero, no key cache is used. The key cache also is not used if the key_buffer_size value is too small to allocate the minimal number of block buffers (8).

If the key cache is not used, index files are accessed using only the native filesystem buffering provided by the operating system. (That is, table index blocks are accessed using the same strategy as that employed for table data blocks.)

An index block is a contiguous unit of access to the MyISAM index files. Usually the size of an index block is equal to the size of nodes of the index B-tree. (Indexes are represented on disk using a B-tree data structure. Nodes at the bottom of the tree are leaf nodes. Nodes above the leaf nodes are non-leaf nodes.)

All block buffers in a key cache structure are the same size. This size can be equal to, greater than, or less than the size of a table index block. Usually one these two values is the multiple of the other.

When data from any table index block must be accessed, the server first checks whether it is available in some block buffer of the key cache. If it is, the server accesses data in the key cache rather than on disk. That is, it reads from the cache or writes into it rather than reading from or writing to disk. Otherwise, the server chooses a cache block buffer containing a different table index block (or blocks) and replaces the data there by a copy of required table index block. As soon as the new index block is in the cache, the index data can be accessed.

If it happens that a block selected for replacement has been modified, the block is considered ``dirty.'' In this case, before being replaced, its contents are flushed to the table index from which it came.

Usually the server follows a LRU (Least Recently Used) strategy: When choosing a block for replacement, it selects the least recently used index block. To be able to make such a choice easy, the key cache module maintains a special queue (LRU chain) of all used blocks. When a block is accessed, it is placed at the end of the queue. When blocks need to be replaced, blocks at the beginning of the queue are the least recently used and become the first candidates for eviction.

7.4.6.1 Shared Key Cache Access

Prior to MySQL 4.1, access to the key cache is serialized: No two threads can access key cache buffers simultaneously. The server processes a request for an index block only after it has finished processing the previous request. As a result, a request for an index block not present in any key cache buffer blocks access by other threads while a buffer is being updated to contain the requested index block.

Starting from version 4.1.0, the server supports shared access to the key cache:

Shared access to the key cache allows the server to improve throughput significantly.

7.4.6.2 Multiple Key Caches

Shared access to the key cache improves performance but does not eliminate contention among threads entirely. They still compete for control structures that manage access to the key cache buffers. To reduce key cache access contention further, MySQL 4.1.1 offers the feature of multiple key caches. This allows you to assign different table indexes to different key caches.

When there may be multiple key caches, the server must know which cache to use when processing queries for a given MyISAM table. By default, all MyISAM table indexes are cached in the default key cache. To assign table indexes to a specific key cache, use the CACHE INDEX statement.

For example, The following statement assigns indexes from the tables t1, t2, and t3 to the key cache named hot_cache: 

mysql> CACHE INDEX t1, t2, t3 IN hot_cache;
+---------+--------------------+----------+----------+
| Table   | Op                 | Msg_type | Msg_text |
+---------+--------------------+----------+----------+
| test.t1 | assign_to_keycache | status   | OK       |
| test.t2 | assign_to_keycache | status   | OK       |
| test.t3 | assign_to_keycache | status   | OK       |
+---------+--------------------+----------+----------+

Note: If the server has been built with the ISAM storage engine enabled, ISAM tables use the key cache mechanism. However, ISAM indexes use only the default key cache and cannot be reassigned to a different cache.

The key cache referred to in a CACHE INDEX statement can be created by setting its size with a parameter setting statement or in the server parameter settings. For example: 

mysql> SET GLOBAL keycache1.key_buffer_size=128*1024;

To destroy a key cache, set its size to zero: 

mysql> SET GLOBAL keycache1.key_buffer_size=0;

See section 10.4.2 Structured System Variables for a description of the syntax used for referring to structured key cache system variables.

By default, table indexes are assigned to the main (default) key cache created at the server start-up. When a key cache is destroyed, all indexes assigned to it become assigned to the default key cache again.

For a busy server, we recommend a strategy that uses three key caches:

One reason the use of three key caches be beneficial is that access to one key cache structure does not block access to the others. Queries that access tables assigned to one cache do not compete with queries that access tables assigned to another cache. Performance gains occur for other reasons as well:

7.4.6.3 Midpoint Insertion Strategy

By default, the key cache management system of MySQL 4.1 uses the LRU strategy for choosing key cache blocks to be evicted, but it also supports a more sophisticated method called the midpoint insertion strategy.

When using the midpoint insertion strategy, the LRU chain is divided into two parts: A hot sub-chain and a warm sub-chain. The division point between two parts is not fixed, but the key cache management system takes care that the warm part is not ``too short,'' always containing at least key_cache_division_limit percent of the key cache blocks. The key_cache_division_limit value is a parameter and can be set per key cache.

When an index block is read from a table into the key cache, it is placed at the end of the warm sub-chain. After a certain number of hits (accesses of the block) it is promoted to the hot sub-chain. At present, the number of hits required to promote a block (3) is the same for all index blocks. In the future, we will allow the hit count to depend on the B-tree level of the node corresponding to an index block: Fewer hits will be required for promotion of an index block if it contains a non-leaf node from the upper levels of the index B-tree than if it contains a leaf node.

A block promoted into the hot sub-chain is placed at the end of the chain. The block then circulates within this sub-chain. If the block stays at the beginning of the sub-chain for a long enough time, it is demoted to the warm chain. This time is determined by the key_cache_age_threshold variable of the key cache.

This variable prescribes that, for a key cache containing N blocks, the block at the beginning of the hot sub-chain not accessed within the last N*key_cache_age_threshold/100 hits is to be moved to the beginning of the warm sub-chain. It then becomes the first candidate for eviction, because blocks for replacement always are taken from the beginning of the warm sub-chain.

The midpoint insertion strategy allows you to keep more valued blocks always in the cache. If you prefer to use the plain LRU strategy, leave the key_cache_division_limit variable set to its default value of 100.

The midpoint insertion strategy helps to improve performance when execution of a query that requires an index scan effectively pushes out of the cache all the index blocks corresponding to valuable high level B-tree nodes. To avoid this, you must use a midpoint insertion strategy with the key_cache_division_limit set to much less than 100. Then valuable frequently hit nodes will be preserved in the hot sub-chain during an index scan operation as well.

7.4.6.4 Index Preloading

If there are enough blocks in a key cache to hold blocks of an entire index, or at least the blocks corresponding to its non-leaf nodes, then it makes sense to preload the key cache with index blocks before starting to use it. Preloading allows you to put the table index blocks into a key cache buffer in the most efficient way: By reading the index blocks from disk sequentially.

Without preloading, the blocks still will be placed into the key cache as needed by queries. In this case, however, although the blocks will stay in the cache as there are enough buffers for all of them, they will be fetched from disk in a random order, not sequentially.

To preload an index into a cache, use the LOAD INDEX INTO CACHE statement. For example, the following statement preloads nodes (index blocks) of indexes of the tables t1 and t2: 

mysql> LOAD INDEX INTO CACHE t1, t2 IGNORE LEAVES;
+---------+--------------+----------+----------+
| Table   | Op           | Msg_type | Msg_text |
+---------+--------------+----------+----------+
| test.t1 | preload_keys | status   | OK       |
| test.t2 | preload_keys | status   | OK       |
+---------+--------------+----------+----------+

The IGNORE LEAVES modifier causes only blocks for the non-leaf nodes of the index to be preloaded. Thus, this statement preloads all index blocks from t1, but only blocks for the non-leaf nodes from t2.

If an index has been assigned to a key cache using a CACHE INDEX statement, preloading places index blocks into that cache. Otherwise, the index is loaded into the default key cache.

7.4.6.5 Key Cache Block Size

MySQL 4.1 introduces a new key_cache_block_size variable on a per-key cache basis. This variable specifies the size of the block buffers for a key cache.

This variable is introduced to allow tuning the performance of I/O operations for index files.

The best performance for I/O operations is achieved when the size of read buffers are equal to the size of the native operating system I/O buffers. But setting the size of key nodes equal to the size of I/O buffer does not always ensure the best overall performance. When reading the big leaf nodes the server pulls in a lot of unnecessary data, effectively preventing reading other leaf nodes.

Currently, you cannot control the size of the index blocks in a table. This size is set by the server when the `.MYI' index file is created, depending on the size of the keys in the indexes present in the table definition. In most cases, it is set equal to the I/O buffer size. In the future this will be changed and then key_cache_block_size variable will be fully employed.

7.4.6.6 Restructuring a Key Cache

A key cache can be restructured at any time by updating its parameter values. For example: 

mysql> SET GLOBAL cold_cache.key_buffer_size=4*1024*1024;

If you assign to either the key_buffer_size or key_cache_block_size key cache component a value that differs from the component's currrent value, the server destroys the cache's old structure and creates a new one based on the new values. If the cache contains any dirty blocks, the server saves them to disk before destroying and recreating the cache. Restructuring does not occur if you set other key cache parameters.

When restructuring a key cache, the server first flushes the contents of any dirty buffers to disk. After that, the cache contents become unavailable. However, restructuring does not block queries that need to use indexes assigned to the cache. Instead, the server directly accesses the table indexes using native filesystem caching. Filesystem caching is not as efficient as using a key cache, so although queries will execute, a slowdown can be anticipated. Once the cache has been restructured, it becomes available again for caching indexes assigned to it. The use of filesystem caching for the indexes ceases.

7.4.7 How MySQL Counts Open Tables

When you run mysqladmin status, you'll see something like this: 

Uptime: 426 Running threads: 1 Questions: 11082 Reloads: 1 Open tables: 12

The Open tables value of 12 can be somewhat puzzling if you have only 6 tables.

MySQL is multi-threaded, so there may be many clients issuing queries for a given simultaneously. To minimise the problem with two client threads having different states on the same file, the table is opened independently by each concurrent thread. This takes some memory but will normally increase performance. With ISAM and MyISAM tables, one extra file descriptor is required for the datafile for each client that has the table open. With these table types, the index file descriptor is shared between all threads.

You can read more about this topic in the next section. See section 7.4.8 How MySQL Opens and Closes Tables.

7.4.8 How MySQL Opens and Closes Tables

The table_cache, max_connections, and max_tmp_tables server variables affect the maximum number of files the server keeps open. If you increase one or more of these values, you may run up against a limit imposed by your operating system on the per-process number of open file descriptors. You can increase the open-files limit on many operating systems, though the method varies widely from system to system. Consult your operating system documentation to determine whether it is possible to increase the limit and how to do so.

table_cache is related to max_connections. For example, for 200 concurrent running connections, you should have a table cache size of at least 200 * n, where n is the maximum number of tables in a join. You also need to reserve some extra file descriptors for temporary tables and files.

Make sure that your operating system can handle the number of open file descriptors implied by the table_cache setting. If table_cache is set too high, MySQL may run out of file descriptors and refuse connections, fail to perform queries, and be very unreliable. You also have to take into account that the MyISAM storage engine needs two file descriptors for each unique open table. You can increase the number of file descriptors available for MySQL with the --open-files-limit=# startup option. See section A.2.17 File Not Found.

The cache of open tables will be kept at a level of table_cache entries. The default value is 64; this can be changed with the -O table_cache=# option to mysqld). Note that MySQL may temporarily open even more tables to be able to execute queries.

A not used table is closed and removed from the table cache under the following circumstances:

When the table cache fills up, the server uses the following procedure to locate a cache entry to use:

A table is opened for each concurrent access. This means the table needs to be opened twice if two threads access the same table or if a thread accesses the table twice in the same query (for example, by joining the table to itself). The first open of any table takes two file descriptors; each additional use of the table takes only one file descriptor. The extra descriptor for the first open is used for the index file; this descriptor is shared among all threads.

If you are opening a table with the HANDLER table_name OPEN statement, a dedicated table object is allocated for the thread. This table object is not shared by other threads and is not closed until the thread calls HANDLER table_name CLOSE or the thread dies. See section 13.1.3 HANDLER Syntax. When this happens, the table is put back in the table cache (if the cache isn't full).

You can check if your table cache is too small by checking the mysqld variable Opened_tables. If this is quite big, even if you haven't done a lot of FLUSH TABLES, you should increase your table cache size. See section 13.5.3.3 SHOW STATUS.

7.4.9 Drawbacks to Creating Large Numbers of Tables in the Same Database

If you have many files in a directory, open, close, and create operations will be slow. If you execute SELECT statements on many different tables, there will be a little overhead when the table cache is full, because for every table that has to be opened, another must be closed. You can reduce this overhead by making the table cache larger.

7.5 Optimizing the MySQL Server

7.5.1 System/Compile Time and Startup Parameter Tuning

We start with the system level factors, because some of these decisions must be made very early. In other cases, a quick look at this section may suffice because it not that important for the big gains. However, it is always nice to have a feeling about how much one could gain by changing things at this level.

The default operating system to use is really important! To get the best use of multiple-CPU machines, you should use Solaris (because its threads implementation works really well) or Linux (because the 2.2 kernel has really good SMP support). Also, older Linux kernels have a 2G file-size limit by default. If you have such a kernel and a desperate need for files larger than 2G, you should get the LFS (large file system) patch for the ext2 filesystem. Other filesystems such as ReiserFS and XFS do not have this 2G limitation.

Because we have not run MySQL in production on that many platforms, we advise you to test your intended platform before choosing it, if possible.

Other tips:

7.5.2 Tuning Server Parameters

You can determine the default buffer sizes used by the mysqld server with this command: 

shell> mysqld --help

This command produces a list of all mysqld options and configurable variables. The output includes the default variable values and looks something like this: 

back_log                 current value: 5
bdb_cache_size           current value: 1048540
binlog_cache_size        current value: 32768
connect_timeout          current value: 5
delayed_insert_timeout   current value: 300
delayed_insert_limit     current value: 100
delayed_queue_size       current value: 1000
flush_time               current value: 0
interactive_timeout      current value: 28800
join_buffer_size         current value: 131072
key_buffer_size          current value: 1048540
lower_case_table_names   current value: 0
long_query_time          current value: 10
max_allowed_packet       current value: 1048576
max_binlog_cache_size    current value: 4294967295
max_connections          current value: 100
max_connect_errors       current value: 10
max_delayed_threads      current value: 20
max_heap_table_size      current value: 16777216
max_join_size            current value: 4294967295
max_sort_length          current value: 1024
max_tmp_tables           current value: 32
max_write_lock_count     current value: 4294967295
myisam_sort_buffer_size  current value: 8388608
net_buffer_length        current value: 16384
net_retry_count          current value: 10
net_read_timeout         current value: 30
net_write_timeout        current value: 60
read_buffer_size         current value: 131072
read_rnd_buffer_size     current value: 262144
slow_launch_time         current value: 2
sort_buffer              current value: 2097116
table_cache              current value: 64
thread_concurrency       current value: 10
tmp_table_size           current value: 1048576
thread_stack             current value: 131072
wait_timeout             current value: 28800

If there is a mysqld server currently running, you can see what values it actually is using for the variables by issuing this statement: 

mysql> SHOW VARIABLES;

You can also see some statistics and status indicators for a running server by issuing this statement: 

mysql> SHOW STATUS;

You can find a full description for all variables in the SHOW VARIABLES section in this manual. See section 13.5.3.4 SHOW VARIABLES. For information about status variables, see section 13.5.3.3 SHOW STATUS.

Server variable and status information also can be obtained using mysqladmin: 

shell> mysqladmin variables
shell> mysqladmin extended-status

MySQL uses algorithms that are very scalable, so you can usually run with very little memory. However, if you give MySQL more memory, normally you will also get better performance.

When tuning a MySQL server, the two most important variables to use are key_buffer_size and table_cache. You should first feel confident that you have these set appropriately before trying to change any other variables.

The following examples indicate some typical variable values for different runtime configurations. The examples use the mysqld_safe script and use --name=value syntax to set the variable name to the value value. This syntax is available as of MySQL 4.0. For older versions of MySQL, take the following differences into account:

If you have at least 256M of memory and many tables and want maximum performance with a moderate number of clients, you should use something like this: 

shell> mysqld_safe --key_buffer_size=64M --table_cache=256 \
           --sort_buffer_size=4M --read_buffer_size=1M &

If you have only 128M of memory and only a few tables, but you still do a lot of sorting, you can use something like: 

shell> mysqld_safe --key_buffer_size=16M --sort_buffer_size=1M

If you have little memory and lots of connections, use something like this: 

shell> mysqld_safe --key_buffer_size=512K --sort_buffer_size=100K \
           --read_buffer_size=100K &

Or even this: 

shell> mysqld_safe --key_buffer_size=512K --sort_buffer_size=16K \
           --table_cache=32 --read_buffer_size=8K -O net_buffer_length=1K &

If you are doing a GROUP BY or ORDER BY on tables that are much larger than your available memory, you should increase the value of read_rnd_buffer_size to speed up the reading of rows after sorting operations.

When you have installed MySQL, the `support-files' directory will contain some different `my.cnf' example files, `my-huge.cnf', `my-large.cnf', `my-medium.cnf', and `my-small.cnf', you can use as a base to optimize your system.

If there are very many simultaneous connections, swapping problems may occur unless mysqld has been configured to use very little memory for each connection. mysqld performs better if you have enough memory for all connections.

Note that if you specicy an option on the command line for mysqld or mysqld_safe, it remains in effect only for that invocation of the server. To use the option every time the server runs, put it in an option file.

To see the effects of a parameter change, do something like this: 

shell> mysqld --key_buffer_size=32m --help

Make sure that the --help option is last; otherwise, the effect of any options listed after it on the command line will not be reflected in the output.

7.5.3 How Compiling and Linking Affects the Speed of MySQL

Most of the following tests are done on Linux with the MySQL benchmarks, but they should give some indication for other operating systems and workloads.

You get the fastest executable when you link with -static.

On Linux, you will get the fastest code when compiling with pgcc and -O3. To compile `sql_yacc.cc' with these options, you need about 200M memory because gcc/pgcc needs a lot of memory to make all functions inline. You should also set CXX=gcc when configuring MySQL to avoid inclusion of the libstdc++ library (it is not needed). Note that with some versions of pgcc, the resulting code will only run on true Pentium processors, even if you use the compiler option that you want the resulting code to be working on all x586 type processors (like AMD).

By just using a better compiler and/or better compiler options you can get a 10-30% speed increase in your application. This is particularly important if you compile the SQL server yourself!

We have tested both the Cygnus CodeFusion and Fujitsu compilers, but when we tested them, neither was sufficiently bug free to allow MySQL to be compiled with optimizations on.

When you compile MySQL you should only include support for the character sets that you are going to use. (Option --with-charset=xxx.) The standard MySQL binary distributions are compiled with support for all character sets.

Here is a list of some measurements that we have done:

The MySQL-Linux distribution provided by MySQL AB used to be compiled with pgcc, but we had to go back to regular gcc because of a bug in pgcc that would generate the code that does not run on AMD. We will continue using gcc until that bug is resolved. In the meantime, if you have a non-AMD machine, you can get a faster binary by compiling with pgcc. The standard MySQL Linux binary is linked statically to make it faster and more portable.

7.5.4 How MySQL Uses Memory

The following list indicates some of the ways that the mysqld server uses memory. Where applicable, the name of the server variable relevant to the memory use is given:

ps and other system status programs may report that mysqld uses a lot of memory. This may be caused by thread-stacks on different memory addresses. For example, the Solaris version of ps counts the unused memory between stacks as used memory. You can verify this by checking available swap with swap -s. We have tested mysqld with commercial memory-leakage detectors, so there should be no memory leaks.

7.5.5 How MySQL uses DNS

When a new client connects to mysqld, mysqld spawns a new thread to handle the request. This thread first checks if the hostname is in the hostname cache. If not, the thread attempts to resolve the hostname:

You can disable DNS hostname lookups by starting mysqld with the --skip-name-resolve option. However, in this case you can only use IP numbers in the MySQL grant tables.

If you have a very slow DNS and many hosts, you can get more performance by either disabling DNS lookups with --skip-name-resolve or by increasing the HOST_CACHE_SIZE define (default value: 128) and recompiling mysqld.

You can disable the hostname cache by starting the server with the --skip-host-cache option. To clear the hostname cache, issue a FLUSH HOSTS statement or execute the mysqladmin flush-hosts command.

If you want to disallow TCP/IP connections entirely, start mysqld with the --skip-networking option.

7.5.6 SET Syntax

SET [GLOBAL | SESSION] sql_variable=expression,
    [[GLOBAL | SESSION] sql_variable=expression] ...

SET sets various options that affect the operation of the server or your client.

The following examples shows the different syntaxes one can use to set variables:

In old MySQL versions we allowed the use of the SET OPTION syntax, but this syntax is now deprecated.

In MySQL 4.0.3 we added the GLOBAL and SESSION options and access to most important startup variables.

LOCAL can be used as a synonym for SESSION.

If you set several variables on the same command line, the last used GLOBAL | SESSION mode is used. 

SET sort_buffer_size=10000;
SET @@local.sort_buffer_size=10000;
SET GLOBAL sort_buffer_size=1000000, SESSION sort_buffer_size=1000000;
SET @@sort_buffer_size=1000000;
SET @@global.sort_buffer_size=1000000, @@local.sort_buffer_size=1000000;

The @@variable_name syntax is supported to make MySQL syntax compatible with some other databases.

The different system variables you can set are described in the system variable section of this manual. See section 10.4 System Variables.

If you are using SESSION (the default), the option you set remains in effect until the current session ends, or until you set the option to a different value. If you use GLOBAL, which requires the SUPER privilege, the option is remembered and used for new connections until the server restarts. If you want to make an option permanent, you should set it in an option file. See section 4.3.2 Using Option Files.

To avoid incorrect usage, MySQL will produce an error if you use SET GLOBAL with a variable that can only be used with SET SESSION or if you are not using SET GLOBAL with a global variable.

If you want to set a SESSION variable to the GLOBAL value or a GLOBAL value to the MySQL default value, you can set it to DEFAULT. 

SET max_join_size=DEFAULT;

This is identical to: 

SET @@session.max_join_size=@@global.max_join_size;

If you want to restrict the maximum value to which a server variable can be set with the SET command, you can specify this maximum by using the --maximum-variable-name command line option. See section 5.2.1 mysqld Command-line Options.

You can get a list of most variables with SHOW VARIABLES. See section 13.5.3.4 SHOW VARIABLES. You can get the value for a specific value with the @@[global.|local.]variable_name syntax: 

SHOW VARIABLES like "max_join_size";
SHOW GLOBAL VARIABLES like "max_join_size";
SELECT @@max_join_size, @@global.max_join_size;

Here follows a description of the variables that use a non-standard SET syntax and some of the other variables. The other variable definitions can be found in the system variable section, among the startup options or in the description of SHOW VARIABLES. See section 10.4 System Variables. See section 5.2.1 mysqld Command-line Options. See section 13.5.3.4 SHOW VARIABLES.

AUTOCOMMIT= 0 | 1
If set to 1, all changes to a table will be done at once. To start a multi-command transaction, you have to use the BEGIN statement. See section 13.4.1 START TRANSACTION, COMMIT, and ROLLBACK Syntax. If set to 0 you have to use COMMIT to accept that transaction or ROLLBACK to cancel it. See section 13.4.1 START TRANSACTION, COMMIT, and ROLLBACK Syntax. Note that when you change AUTOCOMMIT mode from 0 to 1, MySQL performs an automatic COMMIT of any open transaction.
BIG_TABLES = 0 | 1
If set to 1, all temporary tables are stored on disk rather than in memory. This will be a little slower, but you will not get the error The table tbl_name is full for big SELECT operations that require a large temporary table. The default value for a new connection is 0 (that is, use in-memory temporary tables). This variable previously was named SQL_BIG_TABLES. In MySQL 4.0, you should normally never need to set this variable, because MySQL automatically converts in-memory tables to disk-based tables as necessary.
CHARACTER SET character_set_name | DEFAULT
This maps all strings from and to the client with the given mapping. Currently the only option for character_set_name is cp1251_koi8, but you can easily add new mappings by editing the `sql/convert.cc' file in the MySQL source distribution. The default mapping can be restored by using a character_set_name value of DEFAULT. Note that the syntax for setting the CHARACTER SET option differs from the syntax for setting the other options.
INSERT_ID = #
Set the value to be used by the following INSERT or ALTER TABLE command when inserting an AUTO_INCREMENT value. This is mainly used with the binary log.
LAST_INSERT_ID = #
Set the value to be returned from LAST_INSERT_ID(). This is stored in the binary log when you use LAST_INSERT_ID() in a command that updates a table. Setting this variable does not update mysql_insert_id().
LOW_PRIORITY_UPDATES = 0 | 1
If set to 1, all INSERT, UPDATE, DELETE, and LOCK TABLE WRITE statements wait until there is no pending SELECT or LOCK TABLE READ on the affected table. This variable previously was named SQL_LOW_PRIORITY_UPDATES.
MAX_JOIN_SIZE = value | DEFAULT
Don't allow SELECT statements that will probably need to examine more than value row combinations or is likely to do more than value disk seeks. By setting this value, you can catch SELECT statements where keys are not used properly and that would probably take a long time. Setting this to a value other than DEFAULT resets the SQL_BIG_SELECTS value to 0. If you set the SQL_BIG_SELECTS value again, the SQL_MAX_JOIN_SIZE variable will be ignored. You can set a default value for this variable by starting mysqld with the --max_join_size=value option. This variable previously was named SQL_MAX_JOIN_SIZE. Note that if a query result already is in the query cache, no result size check is performed, because the result has already been computed and it will not burden the server to send it to the client.
PASSWORD = PASSWORD('some password')
Set the password for the current user. Any non-anonymous user can change his own password!
PASSWORD FOR user = PASSWORD('some password')
Set the password for a specific user on the current server host. Only a user with access to the mysql database can do this. The user should be given in user@hostname format, where user and hostname are exactly as they are listed in the User and Host columns of the mysql.user table entry. For example, if you had an entry with User and Host fields of 'bob' and '%.loc.gov', you would write: 
mysql> SET PASSWORD FOR 'bob'@'%.loc.gov' = PASSWORD('newpass');
Which is equivalent to: 
mysql> UPDATE mysql.user SET Password=PASSWORD('newpass')
    ->                   WHERE User='bob' AND Host='%.loc.gov';
mysql> FLUSH PRIVILEGES;
QUERY_CACHE_TYPE = OFF | ON | DEMAND
QUERY_CACHE_TYPE = 0 | 1 | 2
Set query cache setting for this thread.
Option Description
0 or OFF Don't cache or retrieve results.
1 or ON Cache all results except SELECT SQL_NO_CACHE ... queries.
2 or DEMAND Cache only SELECT SQL_CACHE ... queries.
SQL_AUTO_IS_NULL = 0 | 1
If set to 1 (default), you can find the last inserted row for a table that contains an AUTO_INCREMENT column by using the following construct: WHERE auto_increment_column IS NULL. This is used by some ODBC programs like Access.
SQL_BIG_SELECTS = 0 | 1
If set to 0, MySQL aborts SELECT statements that probably will take a very long time (that is, statements for which the optimizer estimates that the number of of examined rows probably will exceed the value of MAX_JOIN_SIZE). This is useful when an inadvisable WHERE statement has been issued. The default value for a new connection is 1, which allows all SELECT statements. If you set MAX_JOIN_SIZE to a value other than DEFAULT, SQL_BIG_SELECTS will be set to 0.
SQL_BUFFER_RESULT = 0 | 1
SQL_BUFFER_RESULT forces the result from SELECT statements to be put into a temporary table. This will help MySQL free the table locks early and will help in cases where it takes a long time to send the result set to the client.
SQL_LOG_BIN = 0 | 1
If set to 0, no logging is done to the binary log for the client, if the client has the SUPER privilege.
SQL_LOG_OFF = 0 | 1
If set to 1, no logging is done to the standard log for this client, if the client has the SUPER privilege.
SQL_LOG_UPDATE = 0 | 1
If set to 0, no logging is done to the update log for the client, if the client has the SUPER privilege. This variable is deprecated starting from version 5.0.0 and mapped to SQL_LOG_BIN (see section C.1.2 Changes in release 5.0.0 (22 Dec 2003: Alpha)).
SQL_QUOTE_SHOW_CREATE = 0 | 1
If set to 1, SHOW CREATE TABLE quotes table and column names. This is on by default, so that replication of tables with fancy column names will work. section 13.5.3.8 SHOW CREATE TABLE.
SQL_SAFE_UPDATES = 0 | 1
If set to 1, MySQL aborts UPDATE or DELETE statements that do not use a key or LIMIT in the WHERE clause. This makes it possible to catch wrong updates when creating SQL statements by hand.
SQL_SELECT_LIMIT = value | DEFAULT
The maximum number of records to return from SELECT statements. If a SELECT has a LIMIT clause, the LIMIT takes precedence over the value of SQL_SELECT_LIMIT. The default value for a new connection is ``unlimited.'' If you have changed the limit, the default value can be restored by using a SQL_SELECT_LIMIT value of DEFAULT.
TIMESTAMP = timestamp_value | DEFAULT
Set the time for this client. This is used to get the original timestamp if you use the binary log to restore rows. timestamp_value should be a Unix epoch timestamp, not a MySQL timestamp.

7.6 Disk Issues

7.6.1 Using Symbolic Links

You can move tables and databases from the database directory to other locations and replace them with symbolic links to the new locations. You might want to do this, for example, to move a database to a file system with more free space or increase the speed of your system by spreading your tables to different disk.

The recommended way to do this is to just symlink databases to a different disk and only symlink tables as a last resort.

7.6.1.1 Using Symbolic Links for Databases on Unix

On Unix, the way to symlink a database is to first create a directory on some disk where you have free space and then create a symlink to it from the MySQL database directory. 

shell> mkdir /dr1/databases/test
shell> ln -s /dr1/databases/test mysqld-datadir

MySQL doesn't support that you link one directory to multiple databases. Replacing a database directory with a symbolic link will work fine as long as you don't make a symbolic link between databases. Suppose you have a database db1 under the MySQL data directory, and then make a symlink db2 that points to db1: 

shell> cd /path/to/datadir
shell> ln -s db1 db2

Now, for any table tbl_a in db1, there also appears to be a table tbl_a in db2. If one thread updates db1.tbl_a and another thread updates db2.tbl_a, there will be problems.

If you really need this, you must change the following code in `mysys/mf_format.c': 

if (flag & 32 || (!lstat(to,&stat_buff) && S_ISLNK(stat_buff.st_mode)))

to 

if (1)

On Windows you can use internal symbolic links to directories by compiling MySQL with -DUSE_SYMDIR. This allows you to put different databases on different disks. See section 7.6.1.3 Using Symbolic Links for Databases on Windows.

7.6.1.2 Using Symbolic Links for Tables on Unix

Before MySQL 4.0 you should not symlink tables unless you are very careful with them. The problem is that if you run ALTER TABLE, REPAIR TABLE, or OPTIMIZE TABLE on a symlinked table, the symlinks will be removed and replaced by the original files. This happens because these statements work by creating a temporary file in the database directory and replacing the original file with the temporary file when the statement operation is complete.

You should not symlink tables on systems that don't have a fully working realpath() call. (At least Linux and Solaris support realpath()). You can check if your system supports symbolic links by doing SHOW VARIABLES LIKE 'have_symlink'.

In MySQL 4.0, symlinks are fully supported only for MyISAM tables. For other table types, you will probably get strange problems if you try to use symbolic links on files in the operating system with any of the above commands.

The handling of symbolic links for MyISAM tables in MySQL 4.0 works the following way:

SHOW CREATE TABLE doesn't report if the table has symbolic links prior to MySQL 4.0.15. This is also true for mysqldump, which uses SHOW CREATE TABLE to generate CREATE TABLE statements.

Things that are not yet supported:

7.6.1.3 Using Symbolic Links for Databases on Windows

Beginning with MySQL Version 3.23.16, the mysqld-max and mysql-max-nt servers in the MySQL distribution are compiled with the -DUSE_SYMDIR option. This allows you to put a database directory on a different disk by setting up a symbolic link to it. This is similar to the way that symbolic links work on Unix, though the procedure for setting up the link is different.

On Windows, you make a symbolic link to a MySQL database by creating a file that contains the path to the destination directory. Save the file in the data directory using the filename `db_name.sym', where db_name is the database name.

For example, if the MySQL data directory is `C:\mysql\data' and you want to have database foo located at `D:\data\foo', you should create the file `C:\mysql\data\foo.sym' that contains the pathname D:\data\foo\. After that, all tables created in the database foo will be created in `D:\data\foo'. The `D:\data\foo' directory must exist for this to work. Also, note that the symbolic link will not be used if a directory with the database name exists in the MySQL data directory. This means that if you already have a database directory named `foo' in the data directory, you must move it to `D:\data' before the symbolic link will be effective. (To avoid problems, the server should not be running when you move the database directory.)

Note that because of the speed penalty you get when opening every table, we have not enabled this by default even if you have compiled MySQL with support for this. To enable symlinks you should put in your `my.cnf' or `my.ini' file the following entry: 

[mysqld]
symbolic-links

In MySQL 4.0, symbolic links are enabled by default. If you don't need them, you can disable them with the skip-symbolic-links option.

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